In recent years, the popularization of electronic books has made it possible to view a larger number of electronic books on electronic devices such as PCs (personal computers), PDAs (personal digital assistants), and large-sized displays. Along with the popularization of electronic books, services that sell user-editable electronic books whose duplication is restricted have been under development from the viewpoint of protecting the copyrights of and preventing undue duplication (multiple duplications) of electronic books. An example of the services is disclosed in Non-patent Literature 1.
Non-patent Literature 1 discloses a service that sells electronic textbooks, i.e. a category of electronic book. Specifically, the service permits a user (e.g. a teacher) to create his/her own unique material by installing, onto a PC, content such as characters, still images, and moving images contained in an electronic textbook whose duplication is restricted from the viewpoint of copyright protection etc. and by being enabled to edit the content of the electronic textbook. That is, the service strives for copyright protection etc. by restricting, to use in a single PC onto which the content has been installed in advance, (a) content of an electronic textbook whose duplication is restricted and (b) a user's own unique material created by utilizing the content.